Vapor recompression system



W. H. FARNSWORTH VAPOR RECOMPRESSION SYSTEM Nov. 24, 1953 Filed Jan. l5. 194'? 2 Sheets-Sheet l W.y H. FARNSWORTH VAPOR RECOMPRESSION SYSTEM Nov. 24, 1953 2 Sheets-Sheet 2 Filed Jan. .15. 1947 patented Nov. 24, 1953 attrs vAron aEooMPREssIoN sYs'rEM William H. Farnsworth, Manistee, Mich., assigner to Morton Salt Company, Chicago, Ill., a corporation of IllinoisV Application `anuary 15, 1947, Serial No. 722,187

` (ci. s- 21) 3 Claims.

This invention relates, generally, to improvements in the process and apparatus for making grainer pan salt, and it has particular relation to the application cf a vapor recompression cycle in such process whereby large economies in heat consumption may be effected.

The conventional grainer pan process of making salt, and the apparatus therefor, are well known in the art. In this process, hot brine is evaporated at atmospheric pressure over a large surface area of a grainer pan, care being taken to prevent agitation of the brine surface by boiling. Due to this quiescent surface evaporation, characteristic thin, iiaky crystals of salt are formed at the surface which have a relatively large surface area in respect to the mass of salt. As the crystals become sufficiently heavy they settle to the bottom of the grainer pan and are removed by reciprocating rakes. There is a considerable demand for salt made by the grainer process because of its distinctive physical proporties, particularly its rapid rate of solubility.

Surface evaporation at atmospheric pressure, without boiling, is a comparatively ineicient type of evaporation, and for this reason, the grainer pan process results in a higher B. t. u. consumption per pound of salt produced by this method, than is required in making salt in vacuum pans, which make use of a more eicient type of evaporation. A representative ligure for steam consumption in producing a pound of grainer salt according to good currentplant practice is about 4550 B. t. u.

The object of the present invention, generally stated, is the provision of improvements in the grainer pan process of making salut/hereby a large proportion of the total water to be evaporated from the feed brine is evaporated in such an economical manner, making full use of a vapor recompression cycle, as to considerably reduce the overall heat consumption in grainer salt production. As will appear hereinafter, the present representative heat requirement of about 4500 B. t. u./lb. can be reduced by about onehalf.

More specifically, an object of the present invention is the improvement in the process of making grainer pan salt, and apparatus forl carrying out the same, wherein a considerable portion of the Water to be evaporated from the brine, is evaporated in a flash evaporation process with the flash steam vapor being compressed to a higher temperature and utilized in heating additional brine to a temperature at which a predetermined part of it may be suitably flashed 2 into flash steam vapor while the remainder may be vaporized in a grainer pan.

An important object of the invention is the provision of a method and apparatus wherein a substantial portion of the water content of brine for the production of grainer salt is vaporized by a flashing step making use of a vapor compressor to obtain the reduction in pressure for the flashing step and to compress the dashed steam vapors to a higher temperature, with the vapor compressor being driven by power derived from a steam turbine, and wherein exhaust steam from the turbine and the compressed steam from the vapor compressor are fully utilized in heating additional brine to a temperature at which part of it may be suitably flashed into steam vapor while the remainder may be vaporized in a grainer pan.

Other objects of the invention will, in part, be obvious and will in part appear hereinafter.

For a more complete understanding of the nature and scope of the invention reference may now be had to the following detailed descrip- V tion thereof taken in connection with the accomlar to that shown in Figure 1, but wherein power for driving the vapor compressor is obtained from a turbo-generator set of relatively large capacity.

Referring first to Figure l of the drawings, the reference numeral It designates a grainer pan, which may be of conventional design. Reciprocating rakes (not shown) serve to move the settled salt crystals along the bottom of the pan toward the right hand end thereof for discharge up the drain end l l.

Overflow from the grainer pan l0 and raw feed brine are fed into a brine heater l2 through the line i3. The brine on passing through the brine heater I2 may be heated to a temperature of about` 225 F. by heat interchange with live steam, the source of which will be outlined in detail hereinafter.

The heated brine from heater i2 is conducted through line iii to a ilasher I5 which may be so operated as to evaporate approximately 53.5% of the total amount of water to be vaporized from the brine, by a vapor ashing step. The flasher I5 may be maintained under a vacuum of about 11.1 p. s. i. a. by interconnecting the vapor outlet of the flasher i to the suction side of a vapor compressor I6 through a line I1. In order to protect the compressor` It from corrosion, it is good design practice to provide a vapor washer I8 in the line l1 so as to wash out any entrained brine which may pass ofi with the ilash vapor.

The extent to which the brine may be evaporated in the flasher l5 is controlled by the fact that it cannot be concentrated to such an extent as to bring about the formation of salt crystals therein, since such crystals would not have the distinctive physical properties of grainer pan salt. Accordingly, the amount of vapor to be withdrawn from the flasher I5 is that amount which brings the brine to approximate saturation at the temperature at which the brine is discharged from the flasher I5 through the line 20 into the grainer pan Il. The degree of concentration of the brine in the evaporator I5 is controlled by varying the vacuum under which ashing takes place. As the vacuum is increased (i. e. absolute pressure is reduced) more water is ashed off and the temperature and the resultant vapor pressure are lowered. Conversely, as the vacuum is decreased (i. e. absolute pressure is increased) less Vapor is flashed off and the temperature in the asher is increased. The vacuum under which the flasher I5 operates is controlled by the speed of the vapor compressor I6 which serves to produce the suction.

A satisfactory set of operating conditions for the flasher I5 is as follows: Ilhe brine is fed to the flasher I5 at a temperature of about 225 F. A Vacuum of 11.1 p. s, i.a. is maintained on the flasher I5 which results in a concentration of the brine to slightly under the saturation point at the temperature of approximately 214 F. The temperature of the iiash steam vapors leaving flasher I will be about 198 F. Of course, it will be understood that the exact operating conditions are subject to adjustment by the operator within certain limits, depending largely upon local conditions. As stated before, the controlling factor which must be observed is that the brine must not be concentrated in the flasher l5 to the point where salt crystallization is induced.

The vapo-r washer i8 is supplied with condensate Water discharged by the brine heater I2 from a condensate header 2i through line 22. vA liquid level control device 23 is provided to regulate the supply of condensate into the vapor washer It. As the flash steam vapor in line I1 passes through the vapor washer I3, any brine droplets therein are removed so that the flash steam vapor leaving the washer I8 is free of entrained brine. A salinity control 24 is provided on the vapor washer I8 which serves to automatically discharge the washing water when the salinity thereof is increased to a predetermined point, The salinity control 2d may be of the type which operates on the principle of a salt bridge so as to automatically ccntrol a discharge valve 25 in the discharge line 25.

The vapor washing step, making use of the vapor washer I8 and associated control equipment, is a conventional precaution involving known apparatus, and does not constitute a novel feature of the present invention.

The vapor compressor it is preferably of the rotary type and of an efficient design and may be driven by a steam powered prime mover in the form of a back pressure turbine 21 suitably connected in driving relationship with the.

compressor I6. superheated steam for driving the turbine 21 may be supplied at a pressure of about 600 p. s. i. g. and a total temperature of 750. The compressor I6 and turbine 21 may be so operated that the compressed, superheated, steam produced by the compressor I6 is discharged at a pressure of about 20.78 D. s. i. a., While the exhaust from the turbine 21 is discharged at approximately the same pressure.

The compressed steam from vapor compressor I6 and the exhaust from turbine 21 are discharged into a header 2S which connects with the brine heater I2 through a steam line Si?. Preferably, a desuperheater SI is provided in the steam line 30 Which serves to desuperheat the steam from header 2B before it reaches the brine heater I2. As is well known, live or saturated steam is more efcient in heat interchange equipment than superheated steam, because of the greater rate of heat transfer. The desuperheater 3| is supplied with .condensate from the brine heater I2 through line 32.

As will appear more fully, hereinafter, under the specific operating conditions stated above, the steam requirements of the brine heater I2 exceed those supplied by recompressed steam from vapor compressor I6 plus the exhaust from the turbine 21. The additional steam requirements may be added as superheated steam from line 29 connecting with line 3 9, This superheated steam may be furnished directly from a boiler or as the exhaust from other back pressure turbines.

Instead of supplying the additional steam requirements of the system through line 2S, all, or part, of the additional steam may be supplied to the system in the form of excess actuating steam delivered to the tur-bine 21. The actuating steam may be supplied to the turbine 21 at any convenient pressure and temperature so long as the combined volumes of exhaust from turbine 21 and recompressed vapors from compressor I6, do not exceed the condensing capacity of brine heater I2.

When the grainer pan salt plant shown in connection with Figure 1 is operated under the typical operating conditions above mentioned in connection with vthedescription thereof, it is possible to produce grainer pan salt at a heat consumption of approximately 2300 B. t. u. per pound of salt produced.

An eflcient thermo-compressor (steam jet) could 'be used in place of the turbine 21 and compressor IB.

Where it is necessary to generate electrical power for use in connection with a salt plant, as is often-the case, vit will usually be advantageous to drive the vapor .compressor I6 by an electric motor which, in turn, is energized by power gem erated by thepower plant in the form of one or more turbogenerator sets. These units are provided in highly efficient design for operation onl superheated steam.

In Figure 2 of the drawings, a grainer pan plant similar to that shown and described in connection with Figure 1 is shown wherein the vapor compressor I6 is powered from turbogenerator sets by means of an electric `motor 35 connected in driving relationship therewith. A boiler 3G supplies superheated steam to a turbine 31 at a pressure which may be 600 p. s. i. g. and at a total temperature of 750. The turbine 31 is connected in direct driving relationship with a generator 33 which may. for example, be Of- SQQO rilevati 'aecdase S capacity. The exhaustfrom turbine 37, which may be at a pressure of about 175 p. s. i. g. and a temperature of 500 F., is exhausted through line i0 into a second turbine 4l connected in driving relationship "with a generator 42, which may be ci 3500 kilowatt capacity.

The generators 33 and d2 are suitably connected with a power line t3 and the electrical power may be consumed as needed in the plant, a portion of it being utilized for energizing the motor 35. This motor 35 may be of 481 kilowatt capacity if the compressor i6 is to compress 22,330 pounds of vapor per hour from a pressure of 11.1 p. s. i. a to a pressure of 20.78 p. s. i. a.

The exhaust from the turbines 37 and 4l, together with the recompressed steam from the compressor i5, may be supplied to a header it with which the steam line 30 leading to the brine heater i2 is connected. The exhaust from the turbine l may be at a pressure of approximately l0 p. s. i. g. and a temperature of 240 F.

The exhaust steam from the turbines 3l and 5.3i, plus the compressed steam discharged from vapor compressor l, will exceed the steam requirements or" the brine heater l2, and an auxiliary source of steam is not required.

It is usually preferable to drive the vapor compressor it in the manner described in connection with Figure 2, because of the greater operating efficiency of the large capacity turbo-generators. As is well known, greater operating efficiencies are obtained with this large capacity equipment comparison with a smaller turbine, such as turbine 2l in Figure 1, provided that the power produced by these larger units can be suitably consumed. That portion of the steam supplied to header it which is not required for heating feed brine in the heater I2, may be put to such other use around the plant, such as in the various process operations requiring steam.

The efciency of the processes of evaporation carried out in the plants described in connection with Figures 1 and 2 is made possible by the fact that these plants can efficiently utilize exhaust steam from the steam driven prime movers which serve to power the Vapor compressors I6. As indicated above, the nature of the grainer pan process, requires that about one-half of the total amount of water to be evaporated, be evaporated at the brine surface in the grainer pan. It is possible to lower this ratio somewhat so that 53.5% of the water may be vaporized in the flasher I5, with 45.5% being vaporized in the grainer pan i0.

In the systems shown and described in connection with Figures 1 and 2, each pound of recompressed steam from a vapor compressor I0 will in turn evaporate approximately one pound of water in a flasher I5. Thus, by recompressing the flash steam vapor from flasher i5, the latent heat of condensation is preserved for evaporating more brine. This heat transfer is eiected in the brine heater l2 wherein steam is condensed in heating feed brine. However, this recompression of flesh steam vapor is necessarily attended with the production of a certain proportion of exhaust steam, either from back pressure turbine 2i in Figure 1, or from the two turbo-generator sets in Figure 2. Fortunately, this attendant proportion of exhaust steam can be completely consumed in heating brine in heater I2 for subsequent evaporation in grainer pan l0.

The following representative operating data will serve to illustrate the heat consumption relationship in a plant of the type shown in Figure l:

Capacity of plant- 8050 lbs. grainer salt/hr.

53.5% of all water evaporated in system is evap`dl orated in a flash chamber, while 46.5% is evap-4 orated from the surface of a grainer pan.

Total pounds of i'eed brine (26% salt content) /hr.=8050/.26=31,000 lbs.

Total pounds of water evaporated/hr.=3l,000

.74=22,950 lbs.

Pounds of water evaporated/hr.

=22,950 .535=12,280 lbs.

Pounds of water evaporated/hr. in grainer pan =22,950-l2,280==10,670 lbs.

Vapor leaves flasher at 198 lik- 11.1 p. s. i. a. Vapor is compressed to approximately 359 F.-

20.78 p. s. i. a. Useable heat/lb. of fiash steam vapor=ll45-198 :947 B. t. u. Useable heat in flash steam vapor/hr.=947 12,280=11,629,160 B. t. u. Actuating steam 600 p. s. i. g.-750 TT. Actuating steam required/lb. of flash steam vapor :.393 1b. Useable heat in actuating steam/lb. flash steam vapor=.393 (1379-198) :465 B. t. u. Useable heat in actuating steam/hr.=l2,280

X465=5,710,200. Total heat available/hr. from Vapor compressor and driving turbine=11,629,160 5,710,200

in flasher Each lb. of Water evaporated in system requires 1325 B. t. u.

Total heat required/hr. for evaporation- .1325

22,950=30,408,750 B. t. u.

Auxiliary heat required/hr.=30,408,75017,339,-

360=13,069,.39 B. t. u.

Total heat input to system/hr.=30,408,750-11,-

629,160=18,779,590 B. t u.

Heat input required/lb. grainer salt =l8,779,590/8050=2320 B. t. u.

Percent of heat furnished by ash steam vapor Percent of heat furnished by actuating steam Percent of heat furnished by auxiliary steam It will be apparent that this invention is of a nature which permits of a. number of modications in respect to arrangement and choice of equipment and methods of operating the same, as long as the principles of the invention are adhered to. Accordingly, it is intended that all matter described above or shown in the accompanying drawings, be interpreted as illustrative and not in a limiting sense.

What is claimed as new is:

1. In combination with a grainer pan for making grainer salt, a brine heater, a vapor flasher, conduit means interconnecting the outlet side of said heater with the inlet side of said flasher, conduit means interconnecting said flasher with said grainer pan whereby heated, partially concentrated brine from said flasher may be delivered to said grainer pan, conduit means for delivering partially concentrated brine from said grainer pan to said brine heater, a vapor compressor, conduit means interconnecting the vapor outlet side of said flasher with the suction side of said compressor whereby flash steam vapor may be Withdrawn under suction from said flasher, a steam powered prime mover, driving means operatively interconnecting said compressor in driving relationship with said prime mover,

sd 12st mentonedcQpguit megew may be desuperheafte'd b e'fgre its de Said brine heaterl 2. The ,combination called 1011111 claign 1 where.- in Said vapor cbmpeseo 1190i the .itaryjtyia and s ad prime mov'ersfa 020k" i connected in direen driving re1ati91j1S1111)l compressor.

3'.V The 'combination cfa-I lediorA in 91211111 1 Wheren sad'vap "eo'rprs'sor i's of the' rlot'aryftype, said prime mover 19a, turbq-gener 211101 'said dvig means is a electric moti);1 energjzeq'by 92,101 turhQ-generatgr.

' WnmAM EARNSWQRTH.

Name I .Date Rateau et a1. Aug. .28, 1906 Number Re. 12,526

210 Number 'July 28, 1914 Niolel et a1. Nov. 24, 1914 DeBaufre Jan. 23, 1917 DeBaufre Sept. 13, 1921 Josse etal Aug. 8, 1922 Weil Dec. 25, 1923 Suczek May 12, 1925 Hughes et al. July 11, 1933 Oetken Sept.'19, 1933 Noack Dec. 26, 1939 V911' der Pleeg July' 9, 1940 Klen'schmdt Apr.` 21, 1942 Latham', Jr." Nev. 13, 1945 Kirgap May 11,1948 'Chambere Sept. 21, 1948 FQREIGN PATENTS Country Date Great'Britan Feb. 16, 1900 Great Britain Jan. 19, 1948 

